Note: This post is created based on this Stackoverflow Answer.

Occasionally, you may have a type of record that already exists in a database which you wish to read from, but never modify. In these situations, it can be worth adding a little code to mark these types of records as read-only. This will prevent most ActiveRecord data modification methods to raise an ActiveRecord::ReadOnly exception if code attempts to create or update such a record. Note that read only records do not appear to be protected from destruction!

To mark an individual record as readonly, there is an ActiveRecord::Base method available to help with this. This post concerns marking an entire type of model as readonly. Examples of when this may be necessary include integrating with an external or third-party database, migrating content from one table to another, and supporting shared or private data.

There are two approaches I have seen for marking an entire type of record as readonly, affecting every instance.

The first is the one I prefer, as it doesn’t rely on ActiveRecord callbacks:

class MyModel < ApplicationRecord
  # Mark all instances of this type of record as readonly.
  # The logic for this method can be changed if more complicated logic is required.
  def readonly?

If you prefer a simpler implementation and don’t mind callbacks, the following implementation also works:

class MyModel < ApplicationRecord
  after_initialize :readonly!